David Wayne Birdsall created TRAFODION-2667:
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Summary: Incorrect UEC calculation
Key: TRAFODION-2667
URL: https://issues.apache.org/jira/browse/TRAFODION-2667
Project: Apache Trafodion
Issue Type: Bug
Components: sql-cmp
Environment: All
Reporter: David Wayne Birdsall
Assignee: David Wayne Birdsall
When applying a predicate to histogram data for the same column, the resulting
UECs are sometimes incorrect.
ColStatDesc::applySel (optimizer/ColStatDesc.cpp) calls
ColStatDesc::calculateCorrectResultUec to do this UEC calculation. The latter
assumes that the predicate is on a column that is independent of the histogram
column. In the case that the predicate is on the same column as the histogram,
it is clearly quite dependent! Instead, the UEC should be reduced
proportionately in the same manner as the row count.
Though this is a simple change, the consequences may be far-reaching. Plan
changes are likely. Adjustments to Optimizer tuning parameters are likely to be
required. This change, then, should only be done in concert with a larger
tuning effort.
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